2. Field of the Invention
The present invention generally relates to amplifier circuits, and more particularly, to chopper-stabilized amplifiers in which the offset error is corrected.
A differential amplifier provides a voltage at its output which is a function of the difference between the voltages at its inputs. If the inputs are shorted together, the output should be at a predetermined voltage, often at zero volts since there is a zero voltage difference between the inputs. However, because of a condition of imbalance which typically occurs within the amplifier, the output of the amplifier deviates from the expected level. This deviation is known as the amplifier offset voltage. The imbalanced condition within the amplifier may be the result of a variety of causes, such as a mismatch between the input transistors or a mismatch between the load transistors, for example.
If the amplifier is part of a circuit having a negative feedback loop, the offset voltage can be thought of as the additional input voltage necessary to drive the output to the desired level. Accordingly, the input offset voltage generally equals the output offset divided by the overall amplifier gain.
2. Description of the Prior Art
Prior attempts to correct the offset voltage problem have included the development of circuits which utilize a variable resistor to provide an adjustable voltage to one of the amplifier inputs to cancel the input offset voltage. Another method, disclosed in U.S. Pat. No. 4,050,030 to Russell, uses variable resistors to adjust the current between internal stages of the amplifier in order to eliminate the offset voltage. The offset voltage of an amplifier generally does not remain constant, however, and any change in the offset voltage thus requires additional adjustments to the variable resistors.
Other circuits have been developed in an attempt to automatically provide an adjustment to eliminate the offset voltage as the offset varies. These include chopper-stabilized amplifiers which have a separate "chopper" correction circuit intended to automatically correct the offset of the main amplifier. In one class of chopper-stabilized amplifiers, the chopper circuit periodically disconnects the inputs of the amplifier from the input signal and shorts the amplifier inputs together. Any offset appearing at the output is fed to a storage device such as a capacitor. After the inputs are reconnected to the input signal, the voltage on the capacitor is utilized to eliminate the offset voltage. These circuits, such as that shown in U.S. Pat. No. 3,988,689 to Ochi, et al. require the periodic disconnection of the input signal source from the inputs of the amplifier to replenish the capacitor. The disconnection of the amplifier inputs from the input source causes the output signal to be "chopped." The amplifier disclosed in the Ochi patent includes additional circuitry to reduce any discontinuity in the output signal.
Another type of chopper-stabilized amplifier uses a parallel reference amplifier with feedback paths in parallel with the main amplifier. Although this circuit does not disconnect the inputs from the input source, the parallel amplifier significantly increases the cost and complexity of the circuit.
Another problem associated with prior art chopper-stabilized amplifiers is that the circuits could not easily distinguish between DC offsets, which should be corrected for, and small AC input voltages, which the correction circuitry should ignore. For example, in those instances where the AC input signal is at the same frequency as the chopper correction circuit frequency (known as the chopping frequency), previous offset correction circuits have sensed a voltage offset each cycle (which actually is the AC input signal) and have attemped to correct for it by erroneously shifting the voltage output. On the other hand, in the case where the input frequency is somewhat different from the chopping frequency, the circuit might sense a positive voltage (for example) for a few cycles, and then would sense a negative voltage for a few cycles. As a result of the correction circuitry attempting to correct for the AC input signal, the chopper circuitry was erroneously generating a new signal which was the difference between the chopping frequency and the input signal frequency. This is referred to as intermodulation distortion, and can also occur when the input signal is at or near integral multiples of the chopping frequency.
One method of attempting to solve the intermodulation problem has been to filter (attenuate) the input signal to the chopper circuitry occuring near the chopper frequencies. This can reduce the intermodulation problem but only at the expense of inhibiting the chopper circuitry from correcting for actual offsets occurring at the attenuated frequencies.
Still another problem associated with chopper-stabilized amplifiers has been the tendency of the chopper correction circuitry frequency response to add to the response of the main amplifier. As a result, in the region where the AC gain of both are falling (or "rolling off") at 6 db/octave, the composite amplifier AC gain rolls off at 12 db/octave, resulting in instability problems such as oscillation or extreme ringing.